<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>

    <style type="text/CSS">
        .demo0 {
           color: blue;
           font-weight: 100;
        } 
        div {
            color: purple;
        }
        * {
            margin: 0;
            padding: 0;
        }
        span {
            font-size: 100px;
        }
        
        .demo1 {
            text-align: center;
        }

        .blue {
            color: blue;
        }

        .red {
            color: red;
        }
        .yellow {
            color: yellow;
        }
        .green {
            color: green;
        }

    </style>
</head>
<body>
   <div class="demo0">我是类选择器</div>
   <div>我是类选择器</div>
   <div>我是类选择器</div>
   <div>我是类选择器</div>
   <div class="demo0">我是类选择器</div>


   <div class="demo1">
        <span class="blue">G</span>
        <span class="red">o</span>
        <span class="yellow">o</span>
        <span class="blue">g</span>
        <span class="green">l</span>
        <span class="red">e</span>
   </div>


 

</body>
</html>